In our EPC Patent Specification No. 41807 there are disclosed various devices for obtaining a signal which is representative of the mobility of a joint, each of these devices comprising an elastomeric electro-conductive member mounted on an elastomeric substrate and electrically interconnected with a resistance-sensitive electrical network by conductor means. The operation of these devices is such that movement of the joint gives rise to extension and contraction of the elastomeric electro-conductive member as a result of which the electrical resistance of the member varies and this variation is measured by the electrical network. Our EPC Patent Specification No. 89843 discloses various formulations for electro-conductive materials which are suited to use in the devices referred to above.